[Tagging] Accuracy of survey

Warin 61sundowner at gmail.com
Wed Dec 24 04:36:19 UTC 2014

On 24/12/2014 11:29 AM, tagging-request at openstreetmap.org wrote:
> Date: Tue, 23 Dec 2014 17:37:34 +0100
> From: Rainer F├╝genstein <rfu at oudeis.org>
> To: "Tag discussion, strategy and related tools"
> 	<tagging at openstreetmap.org>
> Subject: [Tagging] Accuracy of survey
> Message-ID: <811143140.20141223173734 at oudeis.org>
> Content-Type: text/plain; charset=ISO-8859-1
> mapper A, by means of DGPS, MilStd GPS, crystal ball etc., is able to
> achieve an accuracy of, say, a few centimeters and uses it to add new
> nodes (POIs) to OSM.
> some time later, mapper B with his/her ancestors mechanical GPS device
> (*), achieving an accuracy of max., say, 15 meters, surveys the same
> area, figures out that (by his/her point of view) POIs added by mapper
> A are 15 meters off and corrects their location.
> what is needed here is some tag, saying "don't touch these
> coordinates, they've been surveyed with high(est) accuracy".
> I heard this argument from an pipeline expert, noting that marker
> surveyed with consumer GPS are (for their standards) way off their
> real location.
> maybe this is a non-issue after all, if consensus is that consumer
> GPS accuracy is sufficient enough.
> cu
Terms !!

In Metrology (http://en.wikipedia.org/wiki/Metrology) the words 
accuracy, error, etc have specific meaning ..

The common term 'accuracy' or 'error' may mean a normal Gaussian 
distribution taken at 1 sigma or it could mean a rectangular 
distribution taken at 100% coverage ... probably the semi range.

Does it matter?
To the normal user .. no. They too have a consumer grade GPS .. and that 
too will have errors so an offset will be expected.
The professional surveyor may use OSM . But not as a source for 
performing a survey, but as an indication. They should have access to 
professionally defined points and boundaries, past professional surveys.
So I'd say .. No it does not matter. Provided the relationship to the 
adjacent objects is a reasonable representation of what is there so 
recognition and navigation is possible.

Best tag?
I've seen a node with a note  'move to +32.14342 151.345345' (the 
numbers are fictitious). I did as it said and that conformed much better 
to the Bing sat image than before it was moved. So I left it there. 
Troubling no source was stated, that would have been nice. As would have 
been an 'accuracy' statement. If I have seen a statement of 
'uncertainty' with a 'coverage' factor I'd be surprised!

Best survey? Ideally ?
Several GPS traces done on separate days/weeks/months so as different 
satellite constellations are used. Reference could also be made to any 
professional survey marks (usually administered by the government ,these 
will have known locations and 'accuracies'). These can then be applied 
to a satellite image but there are significant shifts depending on 
topography and parallax errors, so caution is required in there 
application over any significant area.

If you don't want something changed .. add a note and say why ... and 
inculcate the source by adding the source tag!!!! Things that are very 
well documented tend to be left alone.
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